Hitomi observation of the Perseus cluster– temperature and elemental abundances –Shinya Nakashima1 , Hiroya Yamaguchi2 , Kosuke Sato3
Kyoko Matsushita3 , Aurora Simionescu1 , Marshall Bautz4 ,and the Hitomi Collaboration
1

ISAS/JAXA, 2 NASA/GSFC, 3 Tokyo University of Science, 4 MIT/Kavli
E-mail(SN): shinya@astro.isas.jaxa.jp
Abstract

The X-ray emitting hot intracluster medium (ICM) dominates the baryonic mass in clusters of galaxies,
and hence the dynamics and physics of the ICM is crucial to understand the formation and evolution of thegalaxies and clusters. Several clusters have dense cores (the so-called “cool cores”) in which the radiative
cooling time of the ICM is shorter than the age of the universe. However, the observed temperature at
the core region is higher than expected, and moreover the star formation rate of the central galaxy is anorder of magnitude lower than that estimated from the cooling rate. Therefore a heating mechanism isrequired to suppress the cooling of the central dense ICM. Several heating sources are proposed but furtherobservations are necessary to constrain its origin. The metallicity of the ICM is also important because
it stores the metals ever ejected from cluster galaxies. Measurements of metal abundances in the ICM
provides a unique insight into the average star formation history in galaxies.
The Japan’s sixth X-ray astronomy mission, Hitomi (formerly ASTRO-H), was launched on 17th February 2016. The non-dispersive Soft X-ray Spectrometer (SXS) on board Hitomi achieves spectral resolutionof E/∆E ∼ 1200 at 6 keV in orbits, which is about 30 times higher than that of X-ray CCDs. It provides
the first opportunity to determine precise ionization sate, metal abundances, and turbulence velocity of
the ICM utilizing emission line diagnostics. Even though the operation of Hitomi was discontinued onApril 2016 because of the attitude anomaly, Hitomi observed the Perseus cluster for about 300 ks during
its commissioning phase.
We obtained an excellent spectrum of the ICM in the 2.0-20 keV band (limited energy range is due tothe closed gate valve in the commissioning phase). The He-like Fe Kα complex and the He-like Ni Kα lines
nearby the He-like Fe Kβ lines are successfully resolved. The weak emission lines from He-like Cr and Mn
are also clearly detected. The spectrum is almost represented by a single thin-thermal plasma model withthe temperature of ∼4 keV. However, the ionization temperatures of Si, S, Ar, Ca, and Fe derived fromdetailed line diagnostics deviates from a single temperature assumption, and therefore multi-temperatureor non-ionization equilibrium of the ICM is suggested. Indeed a two-temperature plasma model gives abetter fit to the data. The precise plasma temperatures are under investigation because systematic eﬀects
due to the central AGN emission and the calibration uncertainty are not fully understood yet.
We also measured the abundance ratios of major elements to Fe assuming the single temperature plasma
model. All of them are ∼1.0 relative to the solar values, while previous CCD observations reported thesuper-solar abundances of Fe-peaked elements (Cr/Fe, Mn/Fe, and Ni/Fe). The cause of this discrepancy
is under discussion but we believe that our result utilizing the resolved Kα lines is reliable. The abundancesof Fe-peaked elements also shows the diversity of the progenitor of the type Ia supernova.
Key words: galaxies: clusters: individual (Perseus) — galaxies: clusters: intracluster medium —X-rays: galaxies: clusters